JPH04261496A - Method for preventing precipitation of dispersed particle of electroviscous fluid - Google Patents

Abstract

PURPOSE:To prevent precipitation of an electroviscous fluid without making dielectric fine particles hollow. CONSTITUTION:A magnetic field is applied to an electroviscous fluid 3 consisting of 20-40wt.% dielectric fine particles, 25-35wt.% magnetic fine particles and 55-25wt.% dispersing medium to nearly equalize specific gravity of dielectric fine particles with apparent specific gravity of a dispersing medium.

Description

[Detailed description of the invention]

0001

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for preventing precipitation of dielectric fine particles dispersed in an electrorheological fluid.

0002

[Prior Art] Electrorheological fluids are made by dispersing dielectric fine particles such as ion exchange resin and silica in a dispersion medium such as silicone oil, and they rapidly dissipate under the action of an electric field.
Furthermore, it has the property of reversibly increasing its viscosity and becoming a solid state. In order to provide a large amount of force with a small amount of electric power, its use is being developed in many fields such as clutches, shock absorbers, and water pressure valves.

However, while the specific gravity of the dispersion medium is around 0.9, the specific gravity of the dielectric fine particles is as large as around 1.3. Therefore, there is a problem in that phase separation occurs over time and dielectric fine particles precipitate, and it is difficult to disperse the precipitate again. For example, JP-A-1-962
No. 95 discloses an electrorheological fluid in which dispersed particles are hollow bodies and the specific gravity is approximately the same as that of a dispersion medium to prevent precipitation.

0004

[Problem to be Solved by the Invention] However, in the method of preventing precipitation using hollow dispersed particles, it is necessary to control the hollowing rate of the dispersed particles according to the specific gravity of the dispersion medium. The number of steps required to obtain dispersed particles is extremely large. Furthermore, the technology for hollowing dielectric fine particles is not common, and there is also the problem that the cost of hollow dispersed particles is high. Furthermore, the hollow dispersed particles may be destroyed during dispersion, and in such cases, the desired specific gravity cannot be obtained and precipitation occurs.

The present invention has been made in view of the above circumstances, and an object thereof is to make the specific gravity of the dispersed particles similar to that of the dispersion medium without making the dispersed particles hollow.

[0006]

[Means for Solving the Problems] The method for preventing precipitation of dispersed particles of an electrorheological fluid according to the present invention, which solves the above problems, consists of 20 to 40% by weight of dielectric fine particles, 25 to 35% by weight of magnetic fine particles, and the balance being dispersed. It is characterized in that the specific gravity of the dielectric particles and the apparent specific gravity of the dispersion medium are made approximately the same by applying a magnetic field to an electrorheological fluid consisting of a medium.

Colloidal solutions in which ultrafine particles of magnetite, cobalt, etc. are stably dispersed in a dispersion medium such as water or oil are known as magnetic fluids. This magnetic fluid has the property that its apparent viscosity and apparent specific gravity change by applying a magnetic field, and it can be used for vacuum shaft seals, acceleration sensors, etc.
Applications to specific gravity sorting devices, actuators, etc. are being researched, and some have even been commercialized. The present inventor conceived of applying the characteristics of this magnetic fluid to an electrorheological fluid, and completed the present invention as a result of intensive research.

[0008] As the dielectric fine particles, ion exchange resin,
Fine powders of silica, dry zeolite, and other materials used in conventional electrorheological fluids can be used. Generally, particles having a particle size of about 10 μm are used. Magnetite, cobalt, nickel, and the like are known as magnetic particles, and ultrafine particles having a particle size of about 100 Å are generally used.

[0009] The dispersion medium stably holds dielectric particles and magnetic particles in a dispersed state and exhibits both electrorheological fluid and magnetic fluid properties, and includes silicone oil, kerosene oil, spindle oil, etc. Insulating oil can be used. The electrorheological fluid used in the present invention contains 20 to 40% by weight of dielectric fine particles. When the concentration of dielectric fine particles is lower than 20% by weight, the change in shear stress upon application of an electric field is small and the fluid does not function as an electrorheological fluid. Moreover, if it exceeds 40% by weight, the shear stress when no electric field is applied is too large to be put to practical use.

Further, the electrorheological fluid used in the present invention contains 25 to 35% by weight of magnetic fine particles. When the concentration of magnetic fine particles is lower than 25% by weight, the increase in apparent specific gravity upon application of a magnetic field is small and a large magnetic field gradient is required to prevent precipitation. Moreover, when it is higher than 35% by weight, the concentration of dielectric fine particles becomes relatively low, and the shear stress change upon application of an electric field is small, and the function as an electrorheological fluid is deteriorated.

In the method for preventing precipitation of dispersed particles in an electrorheological fluid according to the present invention, a magnetic field is applied to the electrorheological fluid having the above structure. As a result, the specific gravity of the dielectric fine particles and the apparent specific gravity of the dispersion medium containing the remaining magnetic fine particles can be made almost the same, and precipitation of the dielectric fine particles can be prevented. Note that in order to continue to prevent precipitation, it is necessary to continue applying a magnetic field of a constant value.

0012

Effects and Effects of the Invention In the method for preventing precipitation of dispersed particles in an electrorheological fluid according to the present invention, fine magnetic particles are contained in the electrorheological fluid. The apparent density of the magnetic fluid containing the magnetic fine particles changes as shown in equation (1) below by applying a magnetic field. Sd=Se+M・gradH/4πg (1)(
(Sd: apparent density, Se: true density of fluid, M: magnetization of magnetic material, gradH: magnetic field gradient, g: acceleration of gravity) That is, the apparent density increases in proportion to the magnetic field gradient. Therefore, in the present invention, components other than the dielectric fine particles act as a magnetic fluid, and the application of a magnetic field increases the specific gravity based on the above formula. By setting the magnetic field gradient to an appropriate value, the specific gravity becomes almost the same as that of the dielectric fine particles, so that precipitation of the dielectric fine particles can be prevented.

Therefore, according to the method for preventing precipitation of dispersed particles of the present invention, precipitation can be easily prevented by simply applying a magnetic field without the need to make the dielectric fine particles hollow, thereby eliminating the need for a complicated process for making the dielectric particles hollow. No longer needed. Further, since the precipitation prevention effect is unrelated to the shape of the dielectric fine particles, it is easy to control. Furthermore, even if the type of dispersion medium or dielectric particles is changed, precipitation can be prevented simply by changing the value of the applied magnetic field gradient, which eliminates the conventional complicated process of changing the hollowing ratio. becomes unnecessary.

[0014]

[Examples] Hereinafter, the present invention will be explained in detail using examples. (Example) Commercially available magnetic fluid (“Marpo Magna FL40”)
30 parts by weight of dry zeolite powder as dielectric fine particles were added to 70 parts by weight (manufactured by Matsumoto Yushi Pharmaceutical Co., Ltd.) and dispersed with a stirrer to prepare an electrorheological fluid. Since the magnetic fluid used has a composition of 40% by weight of magnetite and the balance of spindle oil, the obtained electrorheological fluid has a composition of 30% by weight of zeolite, 28% by weight of magnetite, and the balance of spindle oil.

[0015] The density of the zeolite used here is 2.0 g.
/cm3, but the true density (Se) of the magnetic fluid, which is a component excluding zeolite, is 1.3 g/cm3, and if left as is, the zeolite will precipitate. Therefore, in this embodiment, as shown in FIG.
The container 3 containing this electrorheological fluid was placed so that its center was located 7 cm from the surface of the container 3. The temperature is 20
℃ constant condition. At this time, the strength of the magnetic field at the position of the container 3 is about 300 Oe, and the magnetization (M) is about 70 Gauss. Also, the magnetic field gradient (gradH) in this magnetic field is approximately 14
It becomes 0 Oe/cm. Substituting these values into equation (1), the apparent density (Sd) of the magnetic fluid, which is a component excluding zeolite, is approximately 2.1 g/cm3, which is approximately the same as the density of zeolite.

[0016] With the magnetic field applied as described above, the container 3
was allowed to stand, and the relationship between the standing time and the zeolite precipitation rate was investigated. The results are shown in Figure 2. As shown in FIG. 5, the precipitation rate was defined as the ratio (a/b) of the height of the zeolite/magnetic fluid mixed layer to the height of the total liquid. (Comparative Example 1) Furthermore, the sedimentation rate of the electrorheological fluid used in the example was measured in the same manner as in the example except that no magnetic field was applied. The results are shown in Figure 2. (Comparative Example 2) A conventionally used electrorheological fluid was prepared by dispersing and mixing 42% by weight of the same dry zeolite powder as in Example and 58% by weight of spindle oil. The sedimentation rate of this electrorheological fluid was also measured in the same manner as in the example except that no magnetic field was applied. The results are shown in Figure 2. (Evaluation) As is clear from FIG. 2, according to the method of the example, almost no precipitate was formed even after standing for 500 hours or more. Comparison with comparative examples reveals that this effect is due to the inclusion of magnetic fluid and the application of a magnetic field.

[0017] Regarding the electrorheological fluids used in the above Examples and Comparative Example 2, changes in current density and shear stress when the electric field strength was changed were investigated. The results are shown in Figures 3 and 4.
Shown below. The current density and shear stress are as shown in Figure 6.
It was measured using a coaxial double cylinder rotational viscometer. Current density is the value obtained by dividing the current flowing through the ground wire by the area of the inner cylinder surface.
Further, the shear stress is a value obtained by dividing the measured torque by the radius of the inner cylinder and the area of the inner cylinder surface.

It can be seen from FIGS. 3 and 4 that the electrorheological fluid used in the present invention is comparable in electrical characteristics to conventional electrorheological fluids.

[Brief explanation of the drawing]

FIG. 1 is an explanatory diagram of a method of applying a magnetic field in an example.

FIG. 2 is a graph showing the relationship between standing time and precipitation rate.

FIG. 3 is a graph showing the relationship between electric field strength and current density.

FIG. 4 is a graph showing the relationship between electric field strength and shear stress.

FIG. 5 is an explanatory diagram of a method for measuring precipitation rate.

FIG. 6 is an explanatory diagram of a method for measuring current density and shear stress.

[Explanation of symbols]

Claims (1)

[Claims] 1. The specific gravity of the dielectric particles and their dispersion are determined by applying a magnetic field to an electrorheological fluid consisting of 20 to 40% by weight of dielectric particles, 25 to 35% by weight of magnetic particles, and the balance being a dispersion medium. A method for preventing precipitation of dispersed particles in an electrorheological fluid, characterized by making the apparent specific gravity of the medium substantially the same.